Use async_trait

[bootrecords]

First-pass implementation that applies async_trait syntax to the Client, Reader and Writer traits in client/mod.rs as well as to the NewContext trait in client/util.rs.

Note that this leaves the explicit Future generics from the Service trait in server/service.rs untouched and also doesn't introduce async syntax on functions like connect[_slave] in client/tcp.rs, service/tcp.rs and service/rtu.rs. I can still try to work out something there if desired.

Closes #82

[uklotzde]

Thank you!